In another comment, someone says the trick here is to do a resumable upload.
The document is covering the sign at the start of the video, so the uploader could have uploaded that section (without any text on the sign in-game), then "resumed" the upload with a different video that contains the URL on the sign and starts after he switches to the Minecraft window.
The document is covering the sign at the start of the video, so the uploader could have uploaded that section (without any text on the sign in-game), then "resumed" the upload with a different video that contains the URL on the sign and starts after he switches to the Minecraft window.